Stabilized electric oscillator



Aug. 7, 1951 Filed Jan. 5, 1946 G. P. DE MENGEL STABILIZED ELECTRIC OSCILLATOR 3 Sheets-Sheet l Potent/5y N 22,4- ZECC) ml enar PM M Current or Res/stance W Aim/Wei;

Aug. 7, 1951 G. P. DE MENGEL STABILIZED ELECTRIC OSCILLATOR 3 Sheets-Sheet 3 Filed Jan. 5, 1946 F/GG Attorney i atenteci Aug. 7,

STABILIZED ELECTRIC OSCILLATOR Gaston Pakenham de Mengel, London, England,

assignor, by mesne assignments, to International Standard Electric Corporation, New York, N. Y., a corporation of Delaware Application January 5, 1946, Serial No. 639,292 In Great Britain February 23, 1945 15 Claims. (Cl. 25036) The present invention relates to arrangements for stabilisin the frequency and output amplitude of electric oscillators.

It is well known that in the ease of oscillation circuits employing electron discharge devices or valves, maintenance of the oscillations at some steady amplitude depends upon the presence of some non-linear impedance element in the circuit, that is an element whose impedance varies with the current flowing through it. In a large number of well known circuits, the valve itself provides the non-linear element, limitation of the amplitude occurring either by the diode action of the control grid or by the curvature of the anode current-grid voltage characteristic. It has been shown that this type of non-linearity is a fundamental cause of changes in oscillation frequency when the valve supply voltages are changed. The effect is probably due to the changes of energy distribution among the harmonies which are the result of the amplitude limitation.

This major cause of instability has been largely removed in the past by the use of a thermally sensitive resistance element such a a thermistor in the circuit associated with the valve for the purpose of controlling the amplitude. In such a case the circuit elements are so proportioned that the valve operates only over the straight portion of its characteristic curve, grid current limitation being carefully avoided. The changes in resistance of the thermal element are relatively slow, and it cannot follow the instantaneous changes in current, so that for any given amplitude of oscillation it acts substantially as an ordinary resistance. Any change in amplitude causes a compensating change in the resistance of the thermal element, and so in this way harmonics are not generated thereby.

Some of the circuits so far used involve a Wheatstone bridge arrangement necessitating the use of transformer at the input and output of the amplifier portion of the circuit; others involve a bridged T null" circuit. The first of these types is suitable only for single frequency Operation, while the second is not suitable for high frequency operation owing to the spurious phase shifts introduced at high frequencies by the input and output capacities of the amplifier.

When a high degree of frequency stability is required, it is, of course essential that changes of frequency do not occur as a result of the changes in resistance of the thermal element. Unless some care is taken in the design of the oscillation circuit this is liable to happen. The present invention is concerned with oscillation circuit arrangements stabilised by thermally sensitive elements in which the resistance changes are prevented irom affecting the frequency, and differing in essential features from either of the types just mentioned. In these new arrangements the interelectrode impedances of the valve or other electron discharge device are caused to form part of the oscillation circuit in such a way that variations of their resistive components do not affect the frequency, which is determined substantially only by the circuit reactances.

According to the invention, there is provided an arrangement for generating electric oscillations 20 comprising an amplifying arrangement having an 25 trodes and between each electrode and its correspending cathode; and means adapted to be controlled by the voltage across one of the said branches for stabilising the amplitude of the oscillations substantially at a specified value.

The invention also provides an arrangement for generating electric oscillations comprising an amplifying arrangement having an input electrode, an output electrode, and a cathode or cathodes; a coupling network having impedance branches connected respectively between the said input and output electrodes and between each and its corresponding cathode; and a temperature dependent resistance element connected in at least one of the said branches in such manner as to stabilise the amplitude of the oscillations substantially at a specified value.

The invention further provides an arrangement for generating electric oscillations comprising an electron discharge device having a cathode electrode, an input electrode and an output electrode, a coupling network having a plurality of impedance branches connectin the said input and output electrodes, a temperature dependent resistance element in at least one of the said branches, each of the inter-electrode imthe'corresponding cathodes, a third impedance branch connecting the said output electrodes together, a temperature dependent resistance 616-,-

' ment connected in the said third branch and means for connecting the output electrode of each device to the input electrodeof the other, the temperature dependent resistance element being adapted to stabilise the amplitude of the oscillations substantially at a specified value.

The invention will be explained with reference v to the accompanying drawings, in which:

Fig. 1 shows a simplified circuit diagram employed to explain the basis of the invention; Fig.2 shows a, schematic. circuit diagram of oiian'v embodiment in accordance with Fig. 1; Fig. 3, shows thermistor curves used to explain the. stabilising action; and

-gFigs. 4, 5, 6 and v7 show schematic circuit di agrams of other embodiments of the invention.

2111' the simplified diagram illustrating the principles of the invention shown in Fig. 1, there is J shown an unbalanced coupling networkof two shunt impedance elements I and 2, and a series element 3 of which the respective impedances are Z1, Z2 and Z3 respectively. An electron discharge deviced is provided with an input elec. trode ii, an output electrode 5 and a common cathode electrode '1. This device has a mutual conductanceg defined as the ratio of the current obtained from the output electrode 6, to the voltagegapplied between the input electrode 5 with respect to the common electrode 1.

The output electrode of, the device t is con nected to the input terminal 8 of the coupling network, and the output terminal 9 thereof is connected to the input electrode 5. The unipotentialfterminal ll of the network is connected to the common electrode 7 of the device t.

. The impedances Z1, Z and Z3 are intended to include the internal impedances between the electrodes of the device d.

In order to explain the conditions for oscillation, it will first be supposed that the connection betweenthe electrodes 9 and 5 is broken Wltl'lOlltIElIlOVillg the internal impedance be tween the electrodes 5' and 6 which acts in parallel with Z3.

Let 1) be an alternating voltage applied between the electrodes 5 and 7. Then it is Well known that the device will behave as though it supplied a current 2' from the electrode 6 to the terminal 8 of the coupling network, where z'=vg. The resulting voltage between the output terminals 9, IU of the coupling network will be V=iZr where Zr is the transfer impedance of the coupling network, and is equal to The condition for the maintenance of oscil-..

lations when the connection between the electrodes 9 and 5 is restored is that In general, all of the Z impedances will con tain both resistive and reactive components. It will be convenient to mak substitutions corresponding to each of th Z impedances in the form 1/Z=1/ R-|-1/9'X (2) and to rewrite Equation in; the form V This equationmust be satisfied after substitutions corresponding to (2) have been made.

There are, of course, a variety of possible circuits which can be designed to fulfil the conditions. When; as in the case of the present inve'ntion, it is desired that the frequency of os- 'cillation shall be determined only by the react' ances of the circuit, so that it is independent of any'of the resistance components or of then it can be shown that since none of the resistance components of the Z impedances can be negativacondition (1) or (3) cannot be satisfied'unless'g is negative. Thus a single ordi' nary triode could not beTused for the device i.

However, it is wellknown that by appropriately arranging a multirid valve an equivalent triode' can be produced having a negative mutual conductance which can be employed for the device 5 in order to fulfil the condition (1).or (3) i Y If further one of the shunt impedances includes a suitably arranged temperature-dependent resistance such as a thermistor, then the oscillation amplitude can bestabilised since any change or amplitude Wouldcause the violation of con dition (1)01 (3) due to the change in the value of the'corresponding Z impedance. It is evident that the direction of variation of the temperature dependent resistance should be such, as to tend to correct anyvariation of the amplitude of the oscillations.

The present invention consists in the employrhent of an oscillation circuit'of the type of Fig. l, and a temperature dependent resistance in the coupling network for "stabilising the oscillations. Although generally when a single, electron discharge device is used, it must have a negative mutual conductance (or an equivalent arrangement), there is a push-pull'type of circuit to be described later which fulfils the con-1 ditionswhile employing two ordinary triodes;

Fig. -2 shows an embodiment of the invention in accordance with Fig. l. The device 5 is a entode valve having a cathode l, a suppressor grid 5 servin as the input electrode, a screen grid 6- serving as the output electrode, a controlgridl l, and' an anode i2. A bias network consisting of two resistances l3 and I4 shunted by a condenser 11 5 is connected in series with the;

be used. The anode i2 is connected to the pos-;

itive terminal H of adirectcurrentscum? hav ing. a voltage E1 through a decoupling resistance 8, a corresponding .by -pas'sicondenser I9 being provided. 1

The impedance Z1 of Fig. 1 comprises two portions, namely a resistance Re corresponding to the internal resistance of the valve between the electrodes 6 and I, and not representing any actual circuit element, and a parallel resonant circuit comprising an inductance 20 shunted by a condenser 2|. A shunt resistance R7 is introduced to represent the effective parallel resistance of the resonant circuit and does not correspond to any actual circuit element.

The impedance Z2 also comprises two portions, namely a resistance R5 representing the internal resistance between the electrodes 5 and 1 of the valve, and not corresponding to any circuit element, and a shunt comprising a resistance 22 connected in series with a directly heated thermistor 23 having a negative temperature coefficient of resistance. Thermistors suitable for this purpose are described, for example in British Pattent Specification No. 545,679 or 555,563 or 557,541.

The impedance Z3 is represented only by a large blocking condenser 24, so that Z3 is substantially zero.

The screen grid 6 is connected through the inductance 20 and a decoupling resistance 25 to the positive terminal 26 of a direct current source having a voltage E2 which should be greater than E1. The by-pass condenser is 21. The two sources of voltage E1 and E2 need not be separate as shown, but might be derived from a single source as is the usual practice.

The oscillations may be obtained from the output terminals 9 and H], which should preferably be connected to an amplifying valve (not shown), or other high impedance load circuit in any convenient manner.

By putting 23:0, condition (1) may be rewritten for Fig. 2 in hte form:

1 1 9 +z+z= substituting z-a rl and L i 2 2 j a it follows that 1 l 'iand 1 l 0 or since 1 1 l tri ia. and

1 1 1 RTFP E it follows that 1 l l 1 Q'i- -l- -I- -l- Where Rt is the combined resistance of the elements 22 and 23. The oscillation amplitude will accordingly adjust itself until the resistance Rt reaches the value given by Equation 5. An increase in oscillation amplitude would decrease Rt thus reducing the transfer impedance Zt of the coupling network, with the result that the oscillatlon amplitude would decrease again. The reverse eifect would occur if the oscillation amplitude should decrease. Thus stabilisation of the amplitude is obtained without making use of the limiting properties of the valve, and thus without producing harmonics with the consequent variable operation which results therefrom.

It is to be noted that the condition X1 determines the frequency of oscillation, and does not involve any of the resistance components of the circuit, or the mutual conductance 9. Thus the changes in Rt which occur during stabilisation do not affect the frequency. Equation 5 also indicates what was stated above, namely that the condition cannot be fulfilled unless a is negative.

The manner in which stabilisation takes place will be understood from the curves of Fig. 3. The ordinates represent the potential across the terminals oi the resistances 22 and/or 23, and the abscissae represent the corresponding current or resistance. The curved marked 23(C) is the well known current-voltage characteristic of a thermistor with a negative temperature coefficient, and exhibits the potential maximum M which occurs at the early part of the curve. The straight line 22(0) is the corresponding current-voltage characteristic for the constant resistance 22, and the curve 22+23(C) obtained by adding the ordinates of the two curves 22(0) and 23(0) is the characteristic curve for the combination of the elements 22 and 23. By suitably choosing the value of the resistance 22, the combined curve may be given a relatively fiat vertical portion with a fairly sharp corner N which occurs just above the maximum M of the curve 23C).

The curve marked 22+23(R) gives the relation between the resistance and the potential difference across the combination of the elements 22 and 23. The resistance has a relatively largev value for zero voltage, which becomes smaller as the voltage increases'owing to the reduction in resistance of the thermistor as it becomes heated. The reduction in resistance is at first slow, but in the voltage region corresponding to the corner N the reduction is very rapid, and so the middle portion PQ of the curve is nearly horizontal. At higher voltages the reduction in resistance is again smaller and the resistance tends towards an asymptotic value equal to the resistance of the element 22.

The circuit elements should preferably be so proportioned that Equation 5 is satisfied for a resistance value of the combination 22 +23 which corresponds to a value S somewhere on the portion PQ of the curve 22+23(R). Then when the valve is first switched on, the thermistor 23 being cold, oscillations will commence since the transfer impedance Zt of the coupling network is at first larger than the final stabilising value. The thermistor then heats up and the resistance falls to the value corresponding to the point S at which Equation 5 is satisfied. The amplitude of the oscillations changes at the same time and stabilises at the voltage corresponding to this value. Any further change in the amplitude would cause the thermistor to heat or cool and its resistance to change in such manner as to restore the amplitude of the oscillations. It will be evident that by choice of the portion PQ of the curve for stabilising, the control will be very close since a large change in resistance results from a very small change in the applied voltage. It is pos-' sible' in this way to arrange for the resistance to change by a factor of about 10 for a change in voltage of about 25% (corresponding to a power output change of about 2 decibels).

It will be understood that should any of the resistances involved in Equation tend to vary for any reason, a compensating change will occur in the resistance of the thermistor, the point S moving along the nearly flat portion PQ. A negligible change in output'voltage occurs, and as explained above, there will be no change in the frequency. 7

The valve 4 and its biassing arrangements should preferably be' chosen so that it is able to oscillate at the level corresponding to the stabilising voltage without overloading, that is so that only the linear portion of the corresponding characteristic curve is used. In this way the limitation of the amplitude of oscillation' is effected by the thermistor above without the introduction of harmonics, and so stability. of both frequency and amplitude is ensured.

It is further to be noted that the oscillations frequency may be changed over a wide range, for example by adjusting the condenser 2|, without producing any appreciable change in the oscillation amplitude or frequency stability, since the resulting changes (if any) in the value of R1 will also be compensated by the thermistor, without affecting the frequency.

Two other embodiments of the invention which Will now be described, satisfy the Equation 3 in a difierent way, the impedance Zi not bein zero this time.

It can easily be shown that if the impedances of the coupling network are chosen so that either X2/R2=X3/R3 (that is, by making the angle of the impedance Z3 the same as that of Z1 or Z2), then the conditions for satisfying Equation 3 reduce to Thus as before, the conditions for determining the frequency depend only on'the reactances of the circuit, and'the thermistor or other temperashunted by a resistance as.

8 vided to separate the suppressor grid 5 and screen grid 6 as regards the polarising potentials. The impedance Z3 comprises another inductance 33 The resistances and 34 are supposed to include the effect of the resistances of the corresponding inductances, and should be chosen so that the phase angles-of the impedances Z2 and Z3 areequal. Either or both of the resistancesmay be unnecessary. 7

The screen grid current is supplied from the positive high tension terminal i through a resistance and through the inductances 29 and 33. A small fraction of the screen grid current also flows through the thermistor 23, but as the resistances of these inductances Will usually be small compared with those of the elements 2 2 and 23, this small fraction of current will most prob ably be insufficient to afiect the performance of the thermistor appreciably. If this should not be the case, a large blocking condenser (not shown) may be connected in series with the thermistor. A by-pass condenser 35% is provided ,to connect the elements 23 and 29 to ground.

It will be noted that the inductances 29 and 33 are each shunted by the correspondinginterelectrode capacity of the valve i. As it is desirable that the phase angles of the impedances Z2 and Z3 should remain the same at all ire-i 'quencies, a small adjustable trimming condenser 3-? is provided to shunt the inductance 29. is-intended that this condenser shall be adjusted so that the ratio of the capacities which effectively shunt the elements '29 and 33 should be in the inverse ratio of their inductances. Since the capacity between the electrodes 5 and ll is likely to be very much smallerthan that between the electrodes 5 andt the necessary adjustment can most probably be obtained with the trimming condenser 3? in the position shown. If not, it can be connected across the element 33 instead, or trimming condensers could be connected across both inductance elements.

The oscillation output is obtained from" the output terminals 9 and it, will be seen that as in the case of Fig. 2, the output terminal 9 is connected to the branch containing the thermistor. The advantage of this arrangement is that the thermistor automatically compensates for changes in the output load resistture dependent resistance, which is included in one of the shunt resistances R, will stabilise the amplitude of oscillation according to Equation 6 without affecting the frequency. 5

Fig. 4 shows an embodiment of the invention in which X2/R2=X3/R3. The elements in' Fig; 4 which are the same as those of Fig. 1 have been given the same designations. In the case of Fig. 4 the valve interelectrode impedance's have been omitted to avoid complicating the figure, but it' may be assumed that they are present as described with reference to Fig. 2. The shuntimpedance Z1 of the coupling network comprises the resistance 22 and thermistor 23' in series shunted by an adjustable condenser 28; The shunt impedance Z2 consists of an inductance 29 shunted by a resistance 30, and a high resistance 3i which serves to connect the suppressor grid 5 to ground. These latter elements are coupled at'the upper end by a larg'eblo'cking condenser 32 which can be regarded as having zero 1m:

pedance atth'e"osciilation'frequency and is'pro ance. In the case of either Fig. 2 or Fig. .e'the output could if desired be taken from the other end of the coupling network, but the arrange ment shown is preferable.

It will be clear that having adjusted the angles of the impedance Z2 and Z3 to equality, the frequency will be that for which the above condition X1+X2+X3=0 is satisfied. The frequency may be changed by adjusting thecon-" denser 28. The amplitude of the oscillation is then determined by the Equation 6, and the various circuit resistances should be chosen as before so that stabilisation occurs over the por tion PQ of the resistance curve of Fig. 3.

Fig. 5 shows a modification of Fig. l in which the phase angles of the impedances Z1. and Z3 are made equal. It will be seen that the elements 25, 3t andti are interchanged with 22, 23 and 28, the resistance 31 being omitted since it is not now, required. The blocking condenser is now directly in series with the elements 33 and 3 3 and so forms part of Za-instead of Z2. Itslefiect will, however, be negligible provided it is'large enough. The arrangement should otherwise fulfil' the same condition as that of Fig. 4. As in the case of Fig. 4, the trimming condenser :aueasec 31 may beconnected inparallel with the inductance 33 instead of 29, or two trimming condensers can be used. The output is taken from 'the terminals 9 and [9 connected to the ther mistor branch of the coupling network.

The arrangements of Figs. 4 and will produce substantially the same results, but one may be more convenient than the other as regards the choice of the thermistor. In Fig. 4, the thermistor stabilises the screen grid voltage, and

.in Fig. 5 it stabilises the suppressor grid voltage.

This means that in the first case the stabilising voltage of the thermistor can be higher than in the second case.

A possible though less convenient modificapedances Z1 or Z2 must include a temperature. .dependent resistance. 1

It is to be noted that two normally operated valves may be arranged effectively as a discharge device having-a negative mutual conductance. An example is shown in Fig. 6, which indicates how the arrangement to the left hand :side of the dotted line'39 of Fig. l or 5 may be modified to employ two ordinary triodes, the elements to the right hand side of the dotted line being unaltered, Fig, 2 can also be modifled in a similar way. In Fig. 6 the two triodes are designated 39 and 40. The two cathodesare designated 1A and 1B and together correspond to the cathode 1 in Fig. 4. The control grid of the valve 39 corresponds to the input electrode 5, and the anode of the valve 40 corresponds to the output electrode 6. The anode 4! of the valve 39 is coupled to the control grid 42 of the valve 40 through a blocking condenser 43 and is supplied with anode current from the high tension terminal I! (Fig. 4) through a parallel resonant circuit comprising an inductance 44 and a condenser 45 and through a decoupling resistance 46, the corresponding bypass condenser being 41. The control grid ,42 is connected toearth through a high resistance 48, and the two cathodes are biassed positively by means of the usual networks 49 and 59. The parallel resonant circuit should be tuned to the desired oscillation frequency, for example by adjusting condenser 45. This tuned circuit may be shunted by a resistance 5| whose value is supposed to include the shunt effective resistance of the inductance 44, and the internal anode resistance of the valve 391 Let g1 and 92 be the mutual conductances of the valves 39 and 40 respectively, both being positive. Then if a voltage 1) be applied to the control grid 5,'the voltage applied to the control grid 42 will be ''0g1R1, where R7 is the value of the parallel combination of the resistances 5i and 48 and the internal anode-cathode resistance of the valve 39 and the internal control grid-cathode resistance of the valve 49. The efiective output current from the anode 6 will accordingly be +vg1g2Rq; so that the effective mutual conductance of the combination from the input electrode 5 to the output electrode 6 will be equal to gig2R7, and has the desired negativesign. I v

It will be seen that the anode cathode capacity of the valve 39 and the control grid cathode ca pacity of the valve are effectively in parallel with the condenser and so they will both be taken into account in the adjustment of the con.- denser 45. Although in Fig. 6 the valves 39 and 49 have been shown as triodes, in practice it will usually be preferable to use pentodes, or tetrodes. In this case the extra grids may be suitably polarised in any well known way. These details have not been shown in order to avoid complicating the figure.

Fig. '7 shows a rather different arrangement according to the invention in which the oscillation requirements are fulfilled without the use of a device having a negative mutual conductance. The circuit is a push-pull arrangement of two triodes 52 and 53 having mutual conductances g1 and g2. The coupling network comprises two shunt impedances Z1 and Z2 repre sented by the inductances 54 and 55 and a series impedance Z3 represented by the resistance 22 and thermistor 23 shunted by the adjusting condenser 28. The thermistor should have a negative temperature coefficient of resistance. The output electrode or anode 6A of the valve 52 is connected to the junction point of impedances Z1 and Z3, and the input electrode or control grid 5A is connected through a blocking condenser 56 to the junction point of Zzvand, Z3. Likewise the anode 6B of the valve 53 "is connected to the junction point of Z2 and Z3, and the control grid, 53 is connected through a blocking condenser 51 to the junction point of Z1 and Z3. The cathodes IA and 1B are connected to earth through a common resistance 58 and the control grids 5A and 5B are connected to earth through respective resistances 59 and 6. Anode current for both valves is supplied from terminal 6| through a resistance 52 and through the respective inductances 54 and 55. The corresponding by-pass condenser is 63.

The oscillations may be taken from the terminals 64 and 65 connected respectively to the two .anodes, if a balanced output is desired. Alternatively, an unbalanced output can be obtained from either of these terminals and the ground terminal 66. In either case it is desirable to couple the output to a high impedance load circuit such as a valve grid circuit.

It is of course understood that each of the Z impedances includes the interelectrode valve impedances with which it is connected in parallel. Thus Z1 and Z2 each include the anodecathode impedance of one valve and the control grid-cathode impedance of the other, and Z3 includes the anode-control gridimpedances of both the valves. The latter being substantially a capacity may be directly included in the adjustable condenser 28.

The two inductances 54 and 55 should preferably be inductively uncoupled, though in some cases they could be coupled without any objection, for example when it is arranged so that Z1=Z2 and 91:92.

It can be shown by determining the distribution of the currents in the coupling network that the condition for the maintenance of oscillations is in which 91 and 92 are the mutual conductances of the triodes 52 and 53, respectively. vAlthough it is not essential, it is preferable to arrange so that for a balanced push-pull circuit of this kind '11 g1=gz=g'and:Z1- =Z2'=Z, in which case the condition (7) simplifies to Thus as before, the frequency is determined by the reactances alone and is not aiTected either byv the circuit resistances or the mutual conductance of the valves." It is also to benoted that the conditions'can be satisfied with a positive of g. The amplitude of the oscillations is controlled by the thermistor 23 in accordance with Equation 10 since itiforms part of Ra.

It can als'o'be shownthat if the conditions are chosen so that so that then the frequency is determined only by the circuit reactances and control of the amplitude by thethermistor does not afiect the frequency, nor will variations the mutual conductances .of the valves.v

It will be evidentthat the conditions for oscillation could also be satisfied by replacing the elements 54 and 55 by condensers and the element 28 by an inductanceany or all of which could be made adjustable in any convenient Way.

In the balanced arrangement of Fig. 7, there is no need to provide a by-pass condenser to shunt I the cathode resistance 53 because there will be no variation in the current flowing through it, since the sum of the cathode currents of the two valves is substantially constant. This is not necessarily true in other cases and then the usual by-pass condenser may be required.

It will be evident that pentodes or tetrodes can be used for the valves 52 ancl53, the additional grid or gridsbeing connected and polarised in one of the well known ways. 7 V 7 It is to be noted that a temperature dependent resistance or thermistor is not the only kind of device which could be used for stabilising the amplitude of the oscillations according tothe invention. .Thus, referring again to Equation 5, this condition could be satisfied,.for example by varying 9 instead of Rt. Thus if the valve 4 in Fig. '2 isof the variable mu type, the voltage across the input'or output of the coupling net work could be rectified and smoothed and applied to the valve i in the manner of an automatic gain control circuit to vary 9 in the proper sense to counteract any change in amplitude. Alternatively'a grid controlled valve could be arranged to act as the variable resistance Rt, and the rectified oscillator voltage could be applied after sufficient smoothing to the grid to control the valve. These principles could be applied to any of the circuits which have been described. I

In any of the arrangements described the thermistor 23 may be replaced by a series and/01? in the claims, therefore, it is to be understood to include a single of several.

What is claimed is:

thermistor or a'cornbination 1. An arrangement for generating electrical oscillations comprising an electronic amplifier having a negative resistance characteristicand comprising input and output circuits, means for sustaining oscillations in said amplifier comprising a coupling circuit for coupling energy from said output to said input circuit, said coupling circuit comprising series and shunt branch reactance and resistance elements so related in values that the frequency of oscillation of said amplifier is dependent. substantially upon the values of said reactance elements, and one of said branches comprising means controlled by the voltage across said one branch for stabilising the amplitude of oscillations of said amplifier.

2. An arrangement according to claim 1, whereinsaid means for'stabilising comprises a resistance element having anegative co-emcient of resistance with temperature.

3. An arrangement according to claim 2, wherein said reactance elements comprise an adjustable tuning element for tuning said coupling circuit to' a desired oscillation frequency.

4. An arrangement according to claim 3, wherein said coupling circuit comprises a 1: network.

5. An arrangement according to claim 4, wherein said stabilising means comprises a shunt branch of said 11' network and at least one of the said other branches comprises an adjustable parallel tuned circuit.

6. An arrangement according to claim 1, wherein said amplifier comprises an electron discharge device having a plurality of control grid electrodes and a cathode electrode, and said input and output circuits each comprise a separate one of said grid electrodes and said cathode electrode.

'7. An arrangement according to claim 1, wherein said amplifier comprises an electron dis charge device of the pentode type, said input circuit comprises a cathode electrode and a suppressor electrode and the said output circuit comprises a cathode electrode and a screen electrode.

8. An arrangement according to claim '7, wherein said coupling circuit comprises a 11- network.

9. An arrangement according to claim- 8, wherein the shunt branch of said 12' network coupled to said screen electrode comprises a parallel tuned circuit, and the shunt branch of said network connected to said suppressor electrode comprises a resistance having a negative coefiicie'nt of resistance with temperature.

10. An arrangement according to claim 9, wherein the series branch of said 1r network comprises an impedance circuit, the phase angle of said impedance circuit being equal to the phase angle of the shunt branch connected to said screen electrode.

11. An arrangement for generating electrical oscillations comprising an electronic amplifierhaving input and output circuits, said amplifier comprising an electron discharge device of the pentode type, said input circuit comprising a cathode electrode and a suppressor electrode, said output circuit comprising a cathode electrode and a screen electrode, means for sustaining oscillations in said amplifier comprising a coupling circuit for coupling energy from said output to said input circuit, said coupling circuit comprising a 1r network having series and shunt branch reactance and resistance elements so related in values that the frequency of oscillation of said amplifier is dependent substantially upon the values of said reactanoe elements, the shunt branch of said 1r network coupled to said screen electrode comprising means having a negative co-eflicient of resistance with temperature for stabilizing the oscillations of said amplifier, the shunt branch of said 1r network coupled to said suppressor electrode comprises a parallel tuned circuit.

12. An arrangement according to claim 11, wherein the series branch of said 1r network comprises an impedance circuit whose phase angle is equal to the phase angle of said shunt branch connected to said suppressor electrode.

13. An arrangement according to claim 1, wherein said amplifier comprises two electron discharge devices each having a plate, control grid, and cathode electrode, said coupling circuit comprising a 11' network having a series branch connected between said plate electrodes and having the shunt branches coupled between separate plate electrodes and a common connection of said cathode electrodes, the grid electrode of each device being coupled to the plate electrode of the other device.

14. An arrangement according to claim 13, wherein said series branch comprises an element having a negative co-efficient of resistance with 14 temperature shunted by an adjustable reactance circuit for controlling the frequency of oscillation, and means for removing energy from said oscillator circuit coupled across said series branch.

15. An arrangement according to claim 1, wherein said coupling circuit comprises a 1r network, one shunt branch of said 1r network com prising an adjustable tuning element for tuning said coupling circuit to a desired oscillation frequency, the other shunt branch of said 1|- net- Work comprising said stabilising means, said amplifier comprising two electron discharge devices coupled in cascade through a parallel resonant circuit, said coupling circuit having one end coupled to the output circuit of one of said electron discharge devices and the other end coupled to the input circuit Of the other of said electron discharge devices, the series branch of said 1r network comprising an impedance circuit having a phase angle equal to the phase angle of a shunt branch of said 1r network.

GASTON PAKENHAM 1m MENGEL.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,066,333 Carrthers Jan. 5, 1937 2,226,561 Herold Dec. 31, 1940 2,258,128 Black Oct. 7, 1941 2,259,945 Velia Oct. 21, 1941 2,341,067 Wise Feb. 8, 1944 2,407,293 Shepherd Sept. 10, 1946 

